Fix '_terms_enum' on docvalue-only keywords fields (#94719) (#94735)

Running a "_terms_enum" request on a keyword field with "index: false"
setting can currently fail with an exception. This change adds missing
testing around the problematic case and fixes it by implementing missing
methods in SortedSetDocValuesTerms.

Closes #94673

Author: cbuescher

docs/changelog/94719.yaml

@@ -0,0 +1,6 @@
+pr: 94719
+summary: Fix '_terms_enum' on docvalue-only keywords fields
+area: Search
+type: bug
+issues:
+ - 94673

server/src/main/java/org/elasticsearch/index/mapper/KeywordFieldMapper.java

@@ -662,22 +662,22 @@ public int getDocCount() throws IOException {
 
             @Override
             public boolean hasFreqs() {
-                throw new UnsupportedOperationException();
+                return false;
             }
 
             @Override
             public boolean hasOffsets() {
-                throw new UnsupportedOperationException();
+                return false;
             }
 
             @Override
             public boolean hasPositions() {
-                throw new UnsupportedOperationException();
+                return false;
             }
 
             @Override
             public boolean hasPayloads() {
-                throw new UnsupportedOperationException();
+                return false;
             }
 
         }

x-pack/plugin/core/src/test/java/org/elasticsearch/xpack/core/termsenum/TermsEnumTests.java

@@ -0,0 +1,112 @@
+/*
+ * Copyright Elasticsearch B.V. and/or licensed to Elasticsearch B.V. under one
+ * or more contributor license agreements. Licensed under the Elastic License
+ * 2.0; you may not use this file except in compliance with the Elastic License
+ * 2.0.
+ */
+
+package org.elasticsearch.xpack.core.termsenum;
+
+import org.elasticsearch.action.support.WriteRequest;
+import org.elasticsearch.common.settings.Settings;
+import org.elasticsearch.plugins.Plugin;
+import org.elasticsearch.test.ESSingleNodeTestCase;
+import org.elasticsearch.test.InternalSettingsPlugin;
+import org.elasticsearch.xcontent.XContentFactory;
+import org.elasticsearch.xpack.core.LocalStateCompositeXPackPlugin;
+import org.elasticsearch.xpack.core.XPackSettings;
+import org.elasticsearch.xpack.core.termsenum.action.TermsEnumAction;
+import org.elasticsearch.xpack.core.termsenum.action.TermsEnumRequest;
+import org.elasticsearch.xpack.core.termsenum.action.TermsEnumResponse;
+
+import java.io.IOException;
+import java.util.Collection;
+import java.util.List;
+
+import static org.elasticsearch.xcontent.XContentFactory.jsonBuilder;
+import static org.hamcrest.Matchers.contains;
+
+public class TermsEnumTests extends ESSingleNodeTestCase {
+
+    @Override
+    protected Collection<Class<? extends Plugin>> getPlugins() {
+        return List.of(LocalStateCompositeXPackPlugin.class, InternalSettingsPlugin.class);
+    }
+
+    @Override
+    protected Settings nodeSettings() {
+        return Settings.builder().put(XPackSettings.SECURITY_ENABLED.getKey(), "false").build();
+    }
+
+    public void testTermsEnumKeywordIndexed() throws Exception {
+        checkTermsEnumKeywords("true");
+    }
+
+    public void testTermsEnumKeywordDocValuesOnly() throws Exception {
+        checkTermsEnumKeywords("false");
+    }
+
+    private void checkTermsEnumKeywords(String indexed) throws Exception {
+        String indexName = "test_" + indexed;
+        // only shard is preferred to increase likelihood of having more than one segment, which goes through a different code path
+        int numshards = randomBoolean() ? 1 : randomIntBetween(1, 5);
+        createIndex(indexName, Settings.builder().put("index.merge.enabled", false).put("number_of_shards", numshards).build());
+
+        client().admin()
+            .indices()
+            .preparePutMapping(indexName)
+            .setSource(
+                XContentFactory.jsonBuilder()
+                    .startObject()
+                    .startObject("_doc")
+                    .startObject("properties")
+                    .startObject("keyword")
+                    .field("type", "keyword")
+                    .field("index", indexed)
+                    .endObject()
+                    .endObject()
+                    .endObject()
+                    .endObject()
+            )
+            .get();
+        ensureGreen();
+
+        indexAndRefresh(indexName, "1", "keyword", "Apple");
+        indexAndRefresh(indexName, "2", "keyword", "Able");
+        indexAndRefresh(indexName, "3", "keyword", "Banana");
+        indexAndRefresh(indexName, "4", "keyword", "Blue");
+        indexAndRefresh(indexName, "5", "keyword", "Cheddar");
+        {
+            TermsEnumResponse response = client().execute(TermsEnumAction.INSTANCE, new TermsEnumRequest(indexName).field("keyword")).get();
+            List<String> terms = response.getTerms();
+            assertEquals(5, terms.size());
+            assertThat(terms, contains("Able", "Apple", "Banana", "Blue", "Cheddar"));
+        }
+        {
+            TermsEnumResponse response = client().execute(
+                TermsEnumAction.INSTANCE,
+                new TermsEnumRequest(indexName).field("keyword").string("A")
+            ).get();
+            List<String> terms = response.getTerms();
+            assertEquals(2, terms.size());
+            assertThat(terms, contains("Able", "Apple"));
+        }
+        {
+            TermsEnumResponse response = client().execute(
+                TermsEnumAction.INSTANCE,
+                new TermsEnumRequest(indexName).field("keyword").string("B").searchAfter("Banana")
+            ).get();
+            List<String> terms = response.getTerms();
+            assertEquals(1, terms.size());
+            assertThat(terms, contains("Blue"));
+        }
+    }
+
+    private void indexAndRefresh(String indexName, String id, String field, String value) throws IOException {
+        client().prepareIndex(indexName)
+            .setId(id)
+            .setSource(jsonBuilder().startObject().field(field, value).endObject())
+            .setRefreshPolicy(WriteRequest.RefreshPolicy.IMMEDIATE)
+            .get();
+    }
+}